This invention relates to resistive films and processes for preparing and using the resistive films, particularly in the field of electrophotography.
In electrophotography, an electrophotographic plate containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the electrophotographic plate to a support such as paper. This imaging process may be repeated many times with reversible photoconductive insulating layers.
In electophotography, there is a common need for inexpensive easily fabricated resistive films in the resistance range of about 10.sup.2 to 10.sup.8 ohms/square and thicknesses in the range of about 1.0 micron to 500 microns. Resistive films are generally made by dispersing conductive materials in an insulating matrix. However, it is difficult accurately and precisely to control the resistance of films within this range due to sudden changes in resistance at the percolation threshold of the conducting components of the films.
There is a need for resistive films which can be prepared with resistances varied over a substantial range. Fabrication of such films has been problematic. Typically, the resistance of the films is changed by varying the quantity of conductive material dispersed in a binder. A greater resistance is achieved by lower loadings of the conductive material. However, very small decreases in loading of conductive materials at the percolation threshold cause dramatic increases in resistance. These increases in resistance are most dramatically seen when the conductive materials are particles. Light loadings of conductive particles in insulating host polymers have been attempted to avoid the dramatic increase in resistance at the percolation threshold. However, this leads to inhomogeneity and difficulty in controlling material parameters. To reduce this effect, various less conductive materials have been used at high loadings, for example, various metal containing particles and various carbon black particles. However, high loadings of particles in a film make the film brittle.
An example of the need for resistive films can be found in corona charging devices, such as scorotrons. The flat scorotron is a current charging device based on a concept by R. W. Gundlach et al, European Patent Publication No. 0-274-895, published July 20, 1988. The device comprises a set of thin conductive lines deposited on a substrate and is used to replace the free-standing corona wire in a typical electrophotographic device. A flat scorotron has a number of advantages over other corona charging devices, such as being easier to clean, less likely to break because of paper misfeed or cleaning, and inexpensive to produce. However, the device suffers from a number of problems. Any differences in the microstructure of the pins cause each pin to form a corona at a slightly different voltage. Once a corona forms at the end of a pin, the voltage drops because the corona sustaining voltage is less than the corona onset voltage. The drop in voltage prevents other pins from, forming a corona. This self-limiting behavior can be overcome by including current limiting resistances between each pin and the bus bar. However, it is difficult to control the resistances because the required resistivity for such devices is at the edge of the percolation threshold for most materials. Any small, local changes in composition result in large changes in resistivities making it difficult to obtain a controlled, uniform resistivity.
Another example of the need for resistive films can be found in document sensing devices in xerographic copying machines. As a document or paper passes between an electrical contacting brush and a resistive film, the resistance of the circuit is changed. A sensing circuit will produce a signal indicative of the presence and position of the paper and the document path may be corrected. See H. Rommelmann et al, Xerox Disclosure Journal 12(2) 81-2 (1987).
Another example of the need for resistive films can be found in simple voltage sensors for electrostatically charged surfaces. A high voltage sensor fabricated with a resistive film bleeds only a small quantity of charge from a surface leaving the charge density nearly unchanged.
U.S. Pat. No. 4,491,536 to Tomoda et al discloses a composition comprising a fluoroelastomer and carbon fibers having a length of 0.1 to 5 millimeters. A volume "resistivity" of 10.sup.-1 to 10.sup.13 ohm-cm can be achieved with the composition. However, a slight increase in the loading of carbon fiber may produce a dramatic increase in volume resistivity of as much as 12 orders of magnitude. Thus, slight inconsistencies in the composition may lead to large changes in resistivity, especially in compositions having about 15-25% parts fibers.
Fibers have been used to achieve conductive compositions. For example, U.S. Pat. No. 4,569,786 to Deguchi discloses an electrically conductive composition comprising metallic and carbon fibers dispersed in a thermoplastic resin. The metallic and carbon fibers have a length of from 0.5 to 10 mm and are provided to impart conductivity to the composition. U.S. Pat. No. 3,406,126 to Litant discloses a conductive synthetic resin composition containing carbon filaments having a length of 1/4 to 3/4 inch in length. U.S. Pat. No. 4,810,419 to Kunimoto et al discloses a shaped electroconductive aromatic imide polymer article comprising an aromatic imide polymer matrix and 10% to 40% by weight of 0.05 to 3.0 mm long carbon fibers.
In these and other references, the emphasis is primarily on achieving highly conductive compositions. The resistivity of these compositions is difficult to control accurately and precisely. If fibers are used in the composition, the resistivity of the fibers may vary from batch to batch. Further, since the fibers are relatively long, the fibers tend to break. Breaks in the fibers result in fewer conductive pathways, leading to problems such as degraded performance of the composition.
There continues to be a need for materials having a proper resistivity which can be selected accurately while avoiding inhomogeneities in resistivity within the films formed from the materials.